1. Field of the Invention
An electrochemically active gas diffusion barrier is disclosed. The electrochemically active barrier may be used in a high-pressure storage tank to prevent hydrogen from diffusing out of the storage tank.
2. State of the Art
Compressed gases, such as natural gas and hydrogen, are being developed as alternative fuels to replace gasoline and diesel fuels. In order to use compressed gases as fuel sources in vehicles, the vehicles are modified or redesigned to use these alternative fuel sources. For instance, conventional storage tanks for gasoline and diesel fuels cannot withstand high pressures associated with the use of compressed gases. A storage tank of a vehicle that runs on such alternative fuels is configured with an internal shape to withstand the high pressures. The storage tank is also limited in its size and external shape by the space, or available storage envelope, under or within the vehicle that is available for mounting the storage tank. In addition, weight of the storage tank is desirably kept to a minimum so that it does not increase the overall weight of the vehicle. Conventional storage tanks for vehicles that run on alternative fuels are bottle-shaped and are mounted to the underside of the vehicle.
A conformable storage tank for compressed natural gas is disclosed in U.S. Pat. No. 5,577,630 to Blair et al. (xe2x80x9cBlairxe2x80x9d), the disclosure of which is incorporated herein by reference. The conformable storage tank is formed from a polymeric liner that is purported to be impermeable to the compressed natural gas stored in the storage tank. The polymeric liner is overwrapped with a composite material to form a reinforcement layer over the polymeric liner. The polymeric liner is a thin layer of a polyamide, a polyethylene, a polypropylene, a polyurethane, or a blend or copolymer of these materials. The composite material is typically a carbon, glass, graphite, aramid, or other fiber bound in a thermoplastic or thermoset epoxy resin. The conformable storage tank has a normal operating pressure to 3,600 pounds per square inch (xe2x80x9cpsixe2x80x9d) and a burst strength of 11,000 psi.
While the polymeric liner of Blair is used to prevent compressed natural gas from diffusing out of the storage tank, this polymeric liner is not useful to store hydrogen under any significant pressure because hydrogen has a significantly higher permeability rate through polymers than natural gas. The permeability rate of hydrogen is up to 80 times greater than that of natural gas, depending on the polymeric material used in the polymeric liner. If hydrogen is stored in a storage tank having a polymeric liner similar to that in Blair, hydrogen pressure is gradually lost as the hydrogen diffuses out of the storage tank. The diffusing hydrogen also affects the bonding between the polymeric liner and the reinforcement layer because the hydrogen weakens the bond between the two layers.
While other materials are less permeable to hydrogen than polymers, these materials are not currently feasible for coating a polymeric liner of a storage tank to prevent the diffusion of hydrogen and, in addition, exhibit other undesirable characteristics for vehicular applications. Metals and ceramics are less permeable to hydrogen than polymers, but have a higher stiffness or modulus value. If metal or ceramic coatings are formed on the polymeric liner, the coatings would crack when the storage tank was pressurized with hydrogen due to the different stiffnesses of the polymeric liner and the coating, causing the more yieldable polymeric liner to flex before the stiffer coating. Amorphous metals or ceramics also have low hydrogen permeability, but have even higher modulus values. Furthermore, these amorphous metals and ceramics are not easily formed in thin coatings. Coating techniques, such as sputtering or chemical vapor deposition, are problematic because sputtering equipment is not available for coating the inside of a storage tank and chemical vapor deposition requires high temperatures that would damage or destroy the polymeric liner. Using metals and ceramics to coat the polymeric liner is further problematic because such materials are heavier than polymers and drastically increase the overall weight of a storage tank.
It would be desirable to store high-pressure hydrogen in a storage tank that is effectively impermeable to hydrogen diffusion. It would be further desirable to reduce the permeability of hydrogen through polymeric liners so that storage tanks currently used to store compressed natural gas may be modified to store hydrogen.
The present invention comprises a hydrogen diffusion barrier to prevent hydrogen from diffusing through a polymer substrate. The hydrogen diffusion barrier comprises three polymer layers and an energy source. The three polymer layers comprise an anode layer, a cathode layer, and an interposed electrolyte layer, which is conductive to protons and impermeable to hydrogen. The hydrogen diffusion barrier uses an electrochemically active structure to prevent the hydrogen from diffusing through the polymer substrate. If hydrogen passes through the electrolyte layer, a catalytic material present adjacent the interface of the electrolyte layer and the anode layer catalyzes an electrochemical reaction to convert the hydrogen to protons and electrons. The protons and electrons are transported to the cathode layer and reacted to form hydrogen. A catalytic material present adjacent the interface of the cathode layer and the electrolyte layer may be used to enhance the formation of hydrogen.
The present invention also comprises a storage tank for storing high-pressure hydrogen. The storage tank comprises a polymer substrate bonded to a reinforcement layer and a hydrogen diffusion barrier bonded to the polymer substrate. The hydrogen diffusion barrier comprises an anode layer and a cathode layer, each of which comprises a polymer material permeable to hydrogen. The hydrogen-diffusion barrier also comprises an energy source and an electrolyte layer comprising a polymer material conductive to protons and impermeable to hydrogen. The electrolyte layer is disposed between the anode layer and the cathode layer and a catalytic material is present adjacent the interface between the electrolyte layer and at least one of the anode layer and the cathode layer, and preferably adjacent both interfaces. The hydrogen diffusion barrier utilizes this electrochemically active structure to prevent hydrogen from diffusing through the polymer substrate and out of the storage tank.
The present invention further comprises a method of preventing hydrogen from diffusing out of a storage tank. The method comprises providing a storage tank having a polymer substrate bonded to a surrounding reinforcement layer and a hydrogen diffusion barrier bonded to and within the polymer substrate. The hydrogen diffusion barrier comprises an anode layer, a cathode layer, an interposed electrolyte layer, and an energy source. The storage tank is pressurized with hydrogen and any hydrogen that diffuses through the electrolyte layer is transported back to the storage tank using an electrochemically active structure including a catalytic material to catalyze a reaction from hydrogen to protons and electrons. The protons and electrons are transported to the cathode layer where they react in the presence of a catalytic material to form hydrogen, which diffuses back into the storage tank. The energy source provides a sufficient amount of energy to transport the electrons to the cathode layer while the protons diffuse through the electrolyte layer back to the cathode layer.
The present invention still further includes a method of fabricating a storage tank to store high-pressure hydrogen. The method comprises providing a polymer substrate and bonding a reinforcement layer to a first surface of the polymer substrate. A hydrogen diffusion barrier susceptible to electrical stimulation to become electrochemically active is formed on a second surface of the polymer substrate. The hydrogen diffusion barrier comprises a polymeric anode layer permeable to hydrogen, a polymeric electrolyte layer conductive to protons and substantially impermeable to hydrogen, and a polymeric cathode layer permeable to hydrogen. The three polymer layers of the hydrogen diffusion barrier may be formed by dip-coating or spray-coating. A catalytic material may be disposed adjacent the interfaces between the polymeric layers to facilitate dissociation of hydrogen into protons and electrons and to facilitate formation of hydrogen therefrom. An energy source is operably coupled to the anode layer and the cathode layer to render the hydrogen diffusion barrier electrochemically active.